This invention relates to processes and apparatus, including novel compressors and expanders, by means of which improved high efficiency vapour cycles such as Carnot heat engine, refrigeration and heat pump cycles can be approximated in actual practice.
In essence, the Carnot heat engine cycle is composed of four ideal processes: (a) isothermal (zero temperature difference) working fluid heat addition at the desired high temperature, (b) isentropic working fluid expansion (work production), (c) isothermal (zero temperature difference) heat rejection at the desired low temperature and (d) isentropic working fluid compression (work absorption).
Carnot refrigeration and heat pump cycle approximations are also possible, as outlined later. For clarity, most of the background discussion which follows is based on the Carnot heat engine cycle.
Until now, the most energy-efficient heat engine cycle, the above-described Carnot cycle, has been considered merely a theoretical basis upon which to evaluate other practical heat engine cycles and real machinery. This is poignantly outlined in the following quotation from the "Mechanical Engineer's Reference Book", Butterworth Publishers, Boston, 11th Edition, 1986:
"The cycle for the ideal heat engine is known as the Carnot cycle, but has little use in real plants as it is not composed of the steam or gas processes which are found suitable for practical machinery." PA0 "The thermal efficiency of the Carnot cycle is of use to the engineer as it gives him the maximum value that he could attain between given temperature limits". PA0 ". . . It is readily evident that the Rankine cycle has a lower efficiency than the Carnot cycle with the same maximum and minimum temperatures as a Rankine cycle, because the average temperature of heat addition is below the temperature of evaporation. The question might well be asked, why choose the Rankine cycle as the ideal cycle? Why not rather select the Carnot cycle? At least two reasons can be given. The first involves the pumping process. Great difficulties are encountered in building a pump that will handle a mixture of liquid and vapour (coming from the low temperature isotherm--the condenser) and deliver only saturated heated liquid (to the high temperature isotherm--the boiler). It is much easier to completely condense the vapour and handle only liquid in the pump, and the Rankine cycle is based upon this fact. The second reason involves superheating the vapour. In the Rankine cycle, the vapour is superheated at constant pressure. In the Carnot cycle, all the heat transfer is at constant temperature, and therefore the vapour is superheated (assuming single-phase working fluid). However, during this process, the pressure must drop, which means that the heat must be transferred to the vapour as it undergoes an expansion process in which work is done. This is also very difficult to achieve in practice. Thus, the Rankine cycle is the ideal cycle that can be approximated in practice".
Partly because the Carnot cycle, until now, could not itself be actualized or closely approximated, other heat engine conversion cycles have been developed. These heat engine cycles have been primarily based upon the actual machinery and working fluids that were available. For example, the Otto cycle is approximated in practice by the spark ignition engine and the Diesel cycle by the compression-ignition engine. The theoretical heat conversion cycle that is most similar to the Carnot cycle is the Rankine cycle; it is approximated in such applications as steam power plants. Consider the following passage from a college thermodynamics text book, "Thermodynamics", G. J. Van Wyler, Editor, J. Wiley & Sons, Publishers, 1962:
The above conclusion, that for practical reasons one must resort to the lower efficiency Rankine heat engine cycle rather than the Carnot cycle, has been a persuasive one and the classical approach to the Carnot cycle has discouraged most people from even attempting to closely approximate this ideal cycle. Similar considerations have applied in respect of refrigeration and heat pump cycles.